Techniques for improving bond pad performance

ABSTRACT

Techniques for bond pad fabrication are provided. In one aspect, a method of forming a bond pad comprises the following steps. At least one alloying element is selectively introduced to at least a portion of at least one surface of the bond pad. The at least one alloying element is diffused into at least a portion of the bond pad through one or more thermal cycles. The at least one alloying element may be selectively introduced to the bond pad by depositing an alloying element layer comprising the at least one alloying element onto the bond pad and patterning and etching at least a portion of the layer.

FIELD OF THE INVENTION

The present invention relates to chip processing and, more particularly,to bond pads and techniques for fabrication thereof.

BACKGROUND OF THE INVENTION

There is presently an increased demand for custom application-specificintegrated circuit (ASIC) chips. As a result, production outputrequirements have increased.

One particular challenge associated with increasing production of ASICchips is the testing that is required to sort out functional chips fromnon-functional chips. Specifically, each chip produced should be testedbefore packaging. However, with the sizes of the chips and associatedcomponents shrinking, testing becomes increasingly more difficult.

Probe stations typically use an array of tungsten wire probes to contactbond pads on the surface of each chip. These probes are held stationaryand the chips to be tested are raised until the probes make properelectrical contact with the bond pads. These probes are used to applythe necessary power, ground and signal inputs and outputs to test thefunctions of a chip.

In order to ensure that a proper electrical contact is made between thebond pad and the probe, it is standard practice to employ a “scrubbing”motion with the probes when contacting the bond pad. This motion,however, causes damage to the bond pad surface, typically rendering thedamaged portions unusable.

As chip sizes decrease, bond pad sizes also decrease. As a result, themarks left by the probes will consume an ever larger fraction of thebond pad surface.

In order to reduce the size and damage of the scrubs marks, probingparameters such as probe force, probe velocity and overdrive have beenvaried. Unfortunately, a trade-off exists between finding the probeparameters that minimize the size and damage of the scrubs marks and thelow contact resistance, e.g., as achieved by a greater probe force,needed to perform proper chip testing. In fact, it has been found thatcurrent probe technology and parameter variations can no longer providesufficiently low contact resistance while at the same time providing lowscrub damage.

It is known that using a copper alloy material, such as aluminum-copperalloy, to form the bond pad will, to some degree, improve the electricalcontact resistance properties of the bond pad. Therefore, bond padstypically comprise aluminum alloyed with small amounts, e.g., less thanor equal to about two atomic percent (at. %), of copper. The amount ofcopper, however, has to be limited to these small amounts because itspresence adversely affects the fabrication of the bond pad.Specifically, higher levels of copper adversely affect dry etchprocesses typically employed to create the bond pad (making it difficultto properly define various features). Further, bulk copper addition tothe bond pad during fabrication can adversely affect the mechanicalproperties of the bond pad. For example, copper concentrations greaterthan four percent can cause a decrease in the yield strength (a measureof the force required to cause a plastic deformation of the material) ofthe bond pad material.

Therefore, techniques for bond pad fabrication that result in robustbond pads, with high yield strength, that sustain little, if any, damageduring testing, yet maintain a low contact resistance would bedesirable.

SUMMARY OF THE INVENTION

Techniques for bond pad fabrication are provided. In one aspect of theinvention, a method of forming a bond pad comprises the following steps.At least one alloying element is selectively introduced to at least aportion of at least one surface of the bond pad. The at least onealloying element is diffused into at least a portion of the bond padthrough one or more thermal cycles. The at least one alloying elementmay be selectively introduced to the bond pad by depositing an alloyingelement layer comprising the at least one alloying element onto the bondpad and patterning and etching at least a portion of the layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming abond pad according to an embodiment of the present invention;

FIG. 2 is a graph illustrating an exemplary temperature-time profileaccording to an embodiment of the present invention;

FIG. 3A is an image illustrating damage to an untreated aluminum bondpad due to probe testing;

FIG. 3B is an image illustrating damage to a treated aluminum bond paddue to probe testing according to an embodiment of the presentinvention;

FIG. 4A is a graph illustrating contact resistance of an untreatedaluminum bond pad; and

FIG. 4B is a graph illustrating contact resistance of a treated aluminumbond pad according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating exemplary methodology 100 for forming abond pad. In step 102 of FIG. 1, chip 118, e.g., an integrated circuitchip, comprising bond pad 120, is provided. Chip 118 may comprise one ormore terminal dielectric regions.

According to an exemplary embodiment, chip 118 is up to about 1.2micrometers (μm) thick, and bond pad 120 comprises aluminum. Aluminum islow in cost, has high conductivity and is easy to process. Bond pad 120may also comprise less than or equal to about two atomic percent (at. %)of an additional element, or elements, e.g., copper alloyed with thealuminum.

It is to be understood that the choice of an aluminum bond pad is merelyexemplary and that any other suitable bond pad materials may beemployed. By way of example only, suitable bond pad materials include,but are not limited to, aluminum, gold, nickel and combinationscomprising at least one of the foregoing materials.

In step 104, alloying element layer 122 is deposited on at least onesurface of chip 118, and over bond pad 120. According to an exemplaryembodiment, the alloying element employed comprises copper, and alloyingelement layer 122 comprises a pure (e.g., at least 99.999 percent pure)copper layer.

It is to be understood that the choice of a copper alloying element ismerely exemplary and that any other suitable alloying elements may beemployed. Suitable alloying elements include, but are not limited to,copper, nickel, cobalt, palladium, silver, gadolinium and combinationscomprising at least one of the foregoing elements. Further, the use of asingle alloying element is also merely exemplary and according to thetechniques presented herein, multiple alloying elements may be employed.For example, aluminum/nickel/gadolinium alloys may be employed.

The term “alloying element,” as used herein, refers to any element thatmay form an alloy with one or more elements of the bond pad material.

Alloying element layer 122 may be deposited onto chip 118 using anysuitable deposition techniques. Suitable deposition techniques include,but are not limited to, blanket deposition techniques, such as vacuumdeposition techniques, e.g., vacuum evaporation or sputtering. Othersuitable deposition techniques include, but are not limited to,electrolytic plating, electroless plating, metallo-organic vapordeposition, atomic layer deposition and other thin film depositiontechniques.

According to an exemplary embodiment, alloying element layer 122 isdeposited on chip 118 as a blanket film having a thickness of up toabout 250 angstroms. The thickness of alloying element layer 122 dependson several factors, such as the desired concentration of alloyingelement in bond pad 120, the total volume of bond pad 120 and the bulkdiffusivity of the alloying element into bond pad 120.

In step 106, standard lithography techniques are used to pattern thesurface of alloying element layer 122, such that the alloying element,or elements, from alloying element layer 122 may be selectivelyintroduced into aluminum bond pad 120. For example, unwanted portions124 and 126 of alloying element layer 122 can be removed, so as tosubstantially prevent alloying element(s) from being introduced tocertain areas of chip 118, e.g., below portions 124 and 126.Specifically, the alloying element should be introduced primarily to thebond pad section of chip 118, e.g., to prevent electrical shorting ordegradation of the material of chip 118.

According to an exemplary embodiment, patterning of alloying elementlayer 122 is achieved using a photosensitive material. Suitablephotosensitive materials include, but are not limited to, photoimagingpolymers, photoresist, photosensitive polyimide (PSPI), dry film resistsand combinations comprising at least one of the foregoing materials.Suitable photoresist materials include those available from the ShipleyCorporation, of Marlborough, Mass. Suitable dry film resists includethose available from DuPont Printed Circuit Materials, of Durham, N.C.

Once patterned, alloying element layer 122 can then be etched. Accordingto one exemplary embodiment, alloying element layer 122 is etched, e.g.,dip etched, using a conventional wet chemical etchant. The choice ofetchant used is determined by the bond pad structure/composition andalloying element used.

By way of example only, when the alloying element comprises copper andthe bond pad comprises aluminum, a one molar (1M) solution of ammoniumperoxydisulfate (NH₄S₂O₈) in ten volume percent (vol. %) sulfuric acid(H₂SO₄) with an organic complexing agent, such as3-amino-1,2,4-triazole, may be used as a wet etchant solution toselectively etch the copper. This etchant is fast (e.g., etches copperat a rate of greater than about six micrometers per minute) and highlyselective.

Other etching techniques may also be employed. By way of example only,reverse current electrolytic techniques and/or chemical-mechanicalpolishing (CMP) techniques may be employed. For example, reverse currentelectrolytic techniques may be used to electropolish away undesiredmaterial.

Next, any chemical etchant present is rinsed off and the photoresist isstripped. As a result, alloying element layer 122, remains above onlythose areas of bond pad 120 wherein diffusion of the alloying element isdesired.

In step 108, bond pad 120 and alloying element layer 122 are heated.Heating serves to diffuse the alloying element from alloying elementlayer 122 into, and alloy with the element, e.g., aluminum, in bond pad120.

According to an exemplary embodiment, bond pad 120 and alloying elementlayer 122 are heated in an annealing oven evacuated of atmosphericgases. Preferably, the annealing oven is supplied with a flowing streamof one or more reducing gasses to substantially reduce, or prevent, anyoxidation of the diffusing alloying element. Suitable reducing gassesfor use in conjunction with the teachings of the present inventioninclude, but are not limited to, one or more of hydrogen in argon (H₂ inAr), carbon monoxide in argon (CO in Ar) and nitric oxide in nitrogen(NO in N₂). According to a particular embodiment, heating is conductedin a vacuum furnace in the presence of five percent H₂ in Ar as areducing gas.

Once inside the oven, bond pad 120 and alloying element layer 122 aresubjected to one or more thermal cycles. For example, according to oneexemplary embodiment, bond pad 120 and alloying element layer 122 aresubjected to at least one thermal cycle with an ultimate temperature,e.g., the highest temperature reached, of 350° C.

As a result of the heating process, at least a portion of the alloyingelement present in alloying element layer 122 diffuses into and alloyswith the element(s) in bond pad 120. The alloying element may diffusethrough substantially the entire thickness of bond pad 120, oralternatively, only a portion of the thickness thereof. For example, thealloying element may diffuse into only a near-surface region of bond pad120, e.g., near-surface region 128 in step 110. See below. An exemplarytemperature-time profile is shown, for example, in FIG. 2, describedbelow.

In step 110, as mentioned above, the alloying element may diffuse onlyinto near-surface region 128 of bond pad 120. According to an exemplaryembodiment, near-surface region 128 of bond pad 120 comprises up toabout a two micrometer region from the surface of bond pad 120. Forexample, near-surface region 128 of bond pad 120 may comprise up toabout a 1.2 micrometer region from the surface of bond pad 120.

According to another exemplary embodiment, as described below,near-surface region 128 may comprise a nanometer-sized region of bondpad 120. For example, near-surface region 128 may comprise up to about a100 nanometer region from the surface of bond pad 120.

Near-surface region 128 of bond pad 120 may comprise greater than orequal to about 90 percent of all the alloying element present in bondpad 120, e.g., greater than or equal to about 90 percent of all copperin bond pad 120 will be in near-surface region 128. For example,near-surface region 128 of bond pad 120 may comprise greater than orequal to about 95 percent of all the alloying element present in bondpad 120.

Thus, according to an embodiment wherein the alloying element is copperand bond pad 120 comprises aluminum, at least a portion of bond pad 120would, after heating, comprise aluminum-copper alloy. Further, the depthof alloying, or how far into bond pad 120 the copper diffuses, may beregulated by the ultimate temperature and/or the time spent at theultimate temperature. For example, increasing the ultimate temperature,while keeping time constant will increase the depth of alloying.Similarly, increasing the time, but keeping the ultimate temperatureconstant will likewise increase the depth of alloying.

In step 112, oxide layer 130 may be deposited on a surface of chip 118.Nitride layer 132 may then be deposited on a surface of oxide layer 130,e.g., opposite chip 118.

According to an exemplary embodiment, oxide layer 130 and nitride layer132 will serve, on a completed chip, as electrical isolation materials,e.g., between various wiring layers and other bond pads. Other suitableelectrical isolation materials may be employed in addition to, or inplace of, one or more of oxide layer 130 and nitride layer 132,including, but not limited to, fluorinated-silica glass (FSG) materials,silicon-based organic materials, hydrogenated silicon oxycarbide (SiCOH)materials and combinations comprising at least one of the foregoingelectrical isolation materials.

According to one exemplary embodiment, oxide layer 130 has a thicknessof from about 4000 angstroms (Å) to about 5000 Å. For example, oxidelayer 130 may have a thickness of about 4500 Å. Nitride layer 132 has athickness of from about 3500 Å to about 4500 Å. For example, nitridelayer 132 may have a thickness of about 4000 Å.

In step 114, a PSPI layer 134 is deposited on a side of nitride layer132, e.g., opposite oxide layer 130. PSPI layer 134 may serve duringetching of oxide layer 130 and nitride layer 132, e.g., to expose bondpad 120.

In step 116, as mentioned above, oxide layer 130 and nitride layer 132are etched so as to expose bond pad 120. Specifically, etching is usedto remove a portion of oxide layer 130, a portion of nitride layer 132and a portion of PSPI layer 134 in area 136, so as to provide access tobond pad 120.

FIG. 2 is a graph illustrating an exemplary temperature-time profilethat may be used in accordance with the instant techniques. In graph200, temperature is plotted as a function of time.

As shown in graph 200, three distinct phases are present. The firstphase, the dehydration phase, is conducted for a duration of about 30minutes and reaches a temperature of about 120° C. The dehydration phaseserves to remove water from the surface of the alloying element layer.Water and/or oxygen molecules present on the alloying element layer, athigh temperatures, e.g., as during the diffusion phase (see below), willconsume the alloying element by forming oxides, e.g., copper oxides,with it. Water and/or oxygen molecules will also diffuse into thealloying element layer/bond pad interface and impede formation of the,e.g., aluminum-copper alloy.

The next phase, the diffusion phase, is conducted for a duration ofabout one hour and reaches a temperature of about 350° C. During thisphase, the atoms of the alloying element acquire enough thermal energyto diffuse into regions of the bond pad (e.g., near-surface region 128of FIG. 1). It is notable that since the treatments to the bond pad arebeing performed while the bond pad is associated with the chip,treatment temperatures should be kept below that which would causedamage to the chip. For example, heat treatment temperatures may be keptbelow about 400° C. to prevent damage to the chip.

The last phase, the cooling phase, is conducted for a duration suitableto bring the oven, e.g., furnace, back to room temperature. The cooldown time and rate of cooling determines how abrupt the diffusion willbe. For example, a bond pad left to cool from about 350° C. back toabout room temperature by natural cooling would likely have the alloyingelement present throughout its entire thickness. Thus, according to oneexemplary embodiment, the bond pad has the alloying element diffusedthroughout its entire thickness.

Depending on the particular application, having the alloying elementthroughout the entire thickness of the bond pad may, however, beundesirable. For example, copper, e.g., as from an alloying elementlayer, diffused through the entire thickness of an aluminum bond pad caninhibit further processing of the bond pad and/or alter its electricalproperties. Thus, in this instance, copper diffusion is preferablylimited to the near-surface region of the bond pad.

Rapid thermal cycles may be employed to abruptly end the diffusion stageand create a more distinct interface between the alloyed region of thebond pad and the rest of the bond pad. For example, an abrupt coolingphase might last for a duration of up to about 20 minutes.

The temperatures and durations shown in FIG. 2 are merely exemplary. Forexample, the particular temperatures employed may, at least in part, bea function of the thermal budget allowed for a given product. By way ofexample only, experiments have shown that copper diffusion can takeplace at temperatures as low as 250° C. Similarly, the particulardurations employed may, at least in part, be a function of thediffusivity of the alloying element into the bond pad. Diffusion timesdecrease exponentially with increasing temperature. Therefore, iftemperatures of about 250° C. rather than, e.g., 350° C., are employed,diffusion times would be exponentially increased.

Further, the thermal cycle shown in FIG. 2, and described herein, issuitable for a standard annealing oven that utilizes natural cooling,e.g., having cooling rates of less than 4° C. per minute. However, oneor more other thermal cycle profiles may be used in accordance with thepresent techniques. For example, one or more rapid thermal annealing(RTA) cycles may be employed to quickly and abruptly change thetemperature of the chip.

The bond pad is then cooled. Further etching may be performed to removeresidual surface alloying element layer, if any remains. Suitableetching techniques include, but are not limited to, the wet chemicaletchant techniques, reverse current electrolytic techniques and CMPtechniques described above. This step is optional. The product may thenproceed to testing and packaging.

One notable advantage of the above bond pad treatment processes is thatthe treated bond pad has a higher strength, as compared to an untreatedbond pad. Specifically, by diffusing an alloying element into at least aportion of the bond pad, the bond pad becomes more resistant to damagecaused by probes during testing of the chip.

For example, FIG. 3A is an image illustrating damage to an untreatedaluminum bond pad due to probe testing and FIG. 3B is an imageillustrating damage to a treated aluminum bond pad due to probe testing.Specifically, the aluminum bond pad in FIG. 3B has been treatedaccording to the present techniques, using copper as the alloyingelement. A comparison of the bond pads shown in FIGS. 3A and 3B revealsthat treatment according to the instant techniques substantially reducestest probe damage to the bond pad surface. Specifically, the scrub marksshown in FIG. 3B are about ten percent smaller, e.g., shorter, thanthose shown in FIG. 3A.

Another notable advantage of the above bond pad treatment process isthat the treated bond pad has a decreased contact resistance, ascompared to an untreated bond pad. FIG. 4A, for example, is a graphillustrating contact resistance of an untreated aluminum bond pad. FIG.4B, by comparison, is a graph illustrating contact resistance of atreated aluminum bond pad. The aluminum bond pad in FIG. 4B has beentreated according to the present techniques, using copper as thealloying element.

Specifically, the graphs shown in FIGS. 4A and 4B show the contactresistance, measured in ohms, as a function of overdrive, measured inmils, for treated and untreated bond pads, respectively, during testingwith a wire probe. Overdrive provides a measure of probe deflection pastinitial contact with the bond pad. For example, when the bond pad ispushed against the probe to make electrical/mechanical contact, theprobe deflects after making initial contact with the bond pad. Overdriveis the measure of this deflection.

As is shown both in the graph in FIG. 4A and the graph in FIG. 4B,contact resistance decreases with increasing overdrive. However, foreach of the overdrive values tested, the graph in FIG. 4B shows thattreatment of the bond pad according to the present techniques markedlydecreases contact resistance, as compared to the untreated bond padrepresented by the graph in FIG. 4A. This decreased contact resistancemeans that during testing a lower probe force may be employed. A lowerprobe force can reduce, or eliminate, damage to the bond pad.

According to the techniques presented herein, the bond pad with analloying element(s) alloyed therein may comprise a nanostructuredmaterial. Specifically, the alloying elements, during diffusion, maysegregate into nanometer-sized grain boundary regions of the bond padmaterial. For example, when the alloying element comprises eithergadolinium or nickel and the bond pad comprises aluminum, the alloyingelements tend to segregate into nanometer-sized boundary regions betweenindividual aluminum grains. This results in a nanostructured material.

Further, as described above, the near-surface region of the bond padinto which the alloying element is diffused, may comprise ananometer-sized region of the bond pad. Thus, a nanostructured bond padwould be formed.

In conclusion, the instant techniques may be employed to improve bondpad performance. Namely, with regard to chip testing, bond pads treatedaccording to the instant techniques exhibit decreased contact resistanceand increased strength. Further, the treated bond pads exhibit improvedadhesion properties when wire bonds are attached thereto.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope or spirit of the invention.

1. A method of forming a bond pad, the method comprising the steps of:selectively introducing at least one alloying element to at least aportion of at least one surface of the bond pad; and diffusing the atleast one alloying element into at least a portion of the bond padthrough one or more thermal cycles.
 2. The method of claim 1, whereinthe step of selectively introducing the at least one alloying element tothe bond pad further comprises the steps of: depositing an alloyingelement layer comprising the at least one alloying element onto the bondpad; patterning at least a portion of the deposited alloying elementlayer; and etching at least a portion of the patterned alloying elementlayer.
 3. The method of claim 1, wherein the at least one alloyingelement is diffused into a near-surface region of the bond pad.
 4. Themethod of claim 1, wherein the at least one alloying element is diffusedinto a near-surface region of the bond pad which comprises up to about atwo micrometer region from a surface of the bond pad.
 5. The method ofclaim 1, wherein the at least one alloying element is diffused into anear-surface region of the bond pad which comprises up to about a 1.2micrometer region from a surface of the bond pad.
 6. The method of claim1, wherein the at least one alloying element comprises one or more ofcopper, nickel, cobalt, palladium, silver and gadolinium.
 7. The methodof claim 1, wherein the bond pad comprises one or more of aluminum, goldand nickel.
 8. The method of claim 2, wherein the an alloying elementlayer is deposited using one or more of vacuum evaporation, sputtering,electrolytic plating, electroless plating, metallo-organic vapordeposition, atomic layer deposition and thin film deposition techniques.9. The method of claim 2, wherein the patterning step comprises use ofone or more of a photosensitive material, a photoimaging polymer, aphotoresist, a photosensitive polyimide and a dry film resist.
 10. Themethod of claim 2, wherein the etching step comprises use of a wetchemical etchant.
 11. The method of claim 2, wherein the etching stepcomprises use of reverse current electrolytic techniques.
 12. The methodof claim 2, wherein the etching step comprises use ofchemical-mechanical polishing techniques.
 13. The method of claim 1,wherein the at least one alloying element is diffused into the at leasta portion of the bond pad through a plurality of thermal cycles.
 14. Abond pad comprising: at least one alloying element present predominatelyin a near-surface region of the bond pad.
 15. The bond pad of claim 14,comprising one or more of aluminum, gold and nickel.
 16. The bond pad ofclaim 14, wherein the at least one alloying element comprises one ormore of copper, nickel, cobalt, palladium, silver and gadolinium. 17.The bond pad of claim 14, wherein the at least one alloying elementcomprises an aluminum/nickel/gadolinium alloy.
 18. The bond pad of claim14, wherein the near-surface region comprises up to about a twomicrometer region from a surface of the bond pad.
 19. The bond pad ofclaim 14, wherein the near-surface region comprises up to about a 1.2micrometer region from a surface of the bond pad.
 20. The bond pad ofclaim 14, wherein the near-surface region comprises greater than orequal to about 90 percent of the at least one alloying element presentin the bond pad.
 21. The bond pad of claim 14, wherein the near-surfaceregion comprises greater than or equal to about 95 percent of the atleast one alloying element present in the bond pad.